1. Field of the Invention
The present invention relates to an injection molding method for molding an article by means of an injection molding machine.
2. Description of the Related Art
Conventional injection molding method adopts such a control as to reduce the injection speed immediately before the mold cavity is filled up and to switch the pressure from the injection pressure to the dwell pressure in response to a signal indicative of the filling up of the mold cavity. Such signals indicative of the filling up of the mold cavity is, for example, a signal indicative of the screw position, a signal indicative of the injection load pressure, a signal indicative of the internal pressure of the runner or cavity or a signal representing the degree of opening of the mold. Alternatively, the injection pressure is adjusted in accordance with the degree of opening of the mold in the dwell pressure application stage of the molding process. Thus, the control of the injection pressure has been done based upon a single factor, although the resin exhibits a complicated behavior immediately before and after the filling up of the mold cavity. Consequently, it has been impossible to simultaneously eliminate defects such as burrs, short shot and sink marks to a satisfactory level.
Referring to FIG. 2, the conventional injection molding method essentially required a wait time in which the filling is continued at a very small rate (low-speed filling mode) under a pressure which is low enough to avoid generation of burrs (see FIG. 4) until the gases in the gas space 11 in the mold cavity 6 are completely replaced with the resin. It is also to be understood that a too early reduction of the pressure from the primary injection pressure or a too early switching to the low-speed filling mode tends to cause a short shot due to increase in the viscosity of the resin at the thin-walled portion 12. In other words, it is not allowed to conduct the reduction in the injection pressure or switching to the low-speed filling mode too early before the filling up of the mold cavity.
When the mold has a plurality of mold cavities as shown in FIG. 3, there may remain a cavity or cavities 6a experiencing a short shot 13 even after other cavities 6a have been filled up. A long wait time is required until the short shot in each such cavity or cavities 6a is completely filled, because such filling has to be done at a pressure low enough to avoid generation of burrs in the cavities 6a which have been completely filled.
Referring to FIGS. 1, 2 and 4, when the mold cavity has been completely filled, the resin has been compressed so that the resin tends to flow back towards the screw 16, as indicated by arrows in FIG. 4. In other words, a reaction force is applied to the screw 16. This phenomenon is often observed when the mode has been switched to the dwell pressure mode. Such reversing of the resin, once it has occurred, makes it difficult to completely fill thin-wall portions of the mold cavity and tends to cause defects such as short shot 13 or sink mark 14. It is therefore necessary to take suitable measure for preventing reverse flow of the resin which is allowed by backward movement of the screw 16.
FIG. 4 shows the state of the resin which has filled up the mold cavity but no shell or solidified surface layer has been formed yet at the resin surface contacting the mold wall, while FIG. 5 shows the state in which the solidified surface layer 10 has been formed. The solidified surface layer 10 starts to be produced from 0.1 to 0.2 second after the state shown in FIG. 4 has been attained. Therefore, possible degree packing, i.e., degree of press-filling, of the resin increases in accordance with the increase in the thickness of the solidified surface layer 10.